The cellular basis for the physiological action of aldosterone, i.e., increased transepithelial sodium transport, will be investigated in the rabbit colon. Methods will be developed for the isolation of highly purified luminal brush border membrane and basolateral membrane vesicles from the epithelial cell layer of this tissue. The effect of previous aldosterone treatment in vitro on sodium transport and membrane protein and lipid turnover and structure will be determined. These studies should enable definition of the changes in membrane metabolism which result in aldosterone-mediated alterations in membrane ion transport.